INT 21 - DOS 1+ - GET ALLOCATION INFORMATION FOR DEFAULT DRIVE

	AH = 1Bh
Return: AL = sectors per cluster (allocation unit)
	CX = bytes per sector
	DX = total number of clusters
	DS:BX -> media ID byte (see #01356)
Notes:	under DOS 1.x, DS:BX points at an actual copy of the FAT; later
	  versions return a pointer to a copy of the FAT's ID byte
	this function may not be properly supported on CD-ROMs and other
	  installable file systems (use AX=4402h"CD-ROM" for CD-ROMs
	  instead)
BUG:	DOS 7.10 (Win95B/Win98) reportedly returns incorrect values in AL and
	  DX for FAT12/FAT16 drives, even though it manages correct results
	  for FAT32 drives (DOS windows are not affected, only real-mode
	  DOS)
SeeAlso: AH=1Ch,AH=36h